The present invention concerns a supporting device for supporting a rolled recording sheet having a core like a tube and a long recording sheet rolled around it. In the present invention, a flange with a supporting axis is attached to the core of the rolled recording sheet. The rolled recording sheet is supported rotatably and detachably via this flange. As the recording sheet, there can be mentioned, for example, a thermosensible paper or a heatsensitive stencil sheet etc. The supporting device for rolled recording sheet in the present invention is used for providing such recording sheet with a recording apparatus. This invention enables the recording sheet to be recorded properly at all times in spite of condition of the flange-attachment to the core.
Generally, in an image forming apparatus such as a printer or a stencil printing apparatus and so on, a recording sheet on which characters or images are formed is contained inside in the form of a roll. For example, the thermosensible paper is used in a printer of a facsimile and so on and the heatsensitive stencil sheet is used in the stencil printing apparatus; these printing sheets are rolled around an axial tube like a paper-made one to form a cylindrical shape. This is named "rolled recording sheet". A flange is attached to each end portion of the axial tube of the rolled recording sheet. The flange is in a disc-form. On the center portion of the outer surface of the flange, a supporting shaft is provided protruding outwardly. On the center portion of the inner surface of the flange, an attachment portion of a cylindrical shape is provided to be inserted into the end portion of the axial tube. When the flange is attached to both the end portion of the axial tube, the centerline of the supporting shaft substantially coincides with that of the rolled recording sheet. Further, in the image forming apparatus, a recording-sheet supporting device with a pair of holders is disposed. The rolled recording sheet is mounted on the pair of the holders of the recording-sheet supporting device via the flanges. The rolled printing sheet is integrated with the flanges, to be supported rotationally and detachably on the recording-sheet supporting device.
In the constitution for supporting the rolled recording sheet thus stated, consideration will be made to a case where the thermosensitive paper or the heatsensitive stencil sheet is recorded by a recording means having a thermal head and a platen roller. If the platen roller rotates, the recording sheet is drawn from the rolled recording sheet. The recording sheet is sandwiched between the thermal head and the platen roller, to be recorded. Now, suppose that the rolled recording sheet is mounted on the holders of the recording-sheet supporting device in such a manner that it can rotate swiftly without any load. In this case, the recording sheet creases when being recorded by being pinched between the thermal head and the platen roller and conveyed. Such phenomenon is often happened when the recording sheet is thin. For example, the heatsensitive stencil sheet is very thin since it is composed of a thermoplastic resin film like a polyester film and a porous substrate such as a Japanese paper or woven fabric. Hence, in the case where the rolled recording sheet is mounted in such a manner that it can rotate without resistance, creasing is likely to occur in the recording sheet when recording is conducted by the thermal head and the platen roller.
In order to solve the problem stated above, a method is proposed as follows: namely, an elastic protrusion is disposed to a position facing the side surface of the flange in the rolled recording sheet supporting device of the image forming apparatus. This elastic protrusion comprises a plate spring and a protrusion attached to the front end of the plate spring. The rolled recording sheet is loaded via the flange when rotating by the protrusion contacting the outer peripheral surface of the flange. The recording sheet is stretched in a direction of drawing after unrolling from the rolled recording sheet, so that the sheet is conveyed to a gap between the thermal head and the platen roller. Namely, the recording sheet is conveyed to the gap with a tension (back-tension) acting in an opposite direction to that of conveying, therefore it is expected that creasing is unlikely to occur.
The flange has the supporting shaft which is to act as a rotating shaft of the rolled recording sheet when the flange attached to the rolled recording sheet is supported by the supporting device. In addition the flange performs the function of preventing the rolled recording sheet from drifting in the axial direction when the rolled recording sheet is attached to the supporting device. The flange also performs the function of preventing the unrolled recording sheet from moving diagonally across the regular direction during conveyance. Further, another function of the flange is to load the rolled recording sheet during rotation by contacting the elastic protrusion.
According to the existent constitution using the elastic protrusion, the protrusion gives a constant load to the rolled recording sheet that is rotating in order to make the back tension constantly act on the recording sheet, so that creasing in the sheet is prevented. In order to give a constant load to the rolled recording sheet during rotation by means of the elastic protrusion using the plate spring, it is necessary to fix the flange at a predetermined position relative to the axial tube. Also, the flange requires contacting the elastic protrusion in a predetermined position. Consequently, the flange should be inserted into the axial tube to reach the regular position when being attached to it. However, there may occur irregularity of the inner diameter and the longitudinal length of the axial tube in manufacturing. Further, if the axial tube is made from paper and humidity is low, the inner diameter may shrink. In these cases, the flange can not be inserted into the axial tube to reach the regular position. Further, if the axial tube is made from paper and humidity is high, the longitudinal length of the axial tube may enlarge so that the axial size with the flanges becomes longer than the required length.
If the flanges are improperly attached to the axial tube, or if the length of the axial tube enlarges, when the rolled recording sheet is mounted on the supporting device via the flanges, the elastic protrusion deforms over the regular value. This causes the contact pressure increase between the flange and the protrusion, so that a force restraining the rotation of the rolled recording sheet enlarges over the regular value. In short, the recording sheet is stretched in a direction of drawing after being drawn from the rolled recording sheet, and the sheet is conveyed to a gap between the thermal head and the platen roller; then, the back tension acting on the recording sheet enlarges excessively. This back tension causes slipping in conveyance of the recording sheet by the platen roller, so that the distance between dots formed on the recording sheet by the thermal head decreases. This brings about a drawback of deteriorating reproductive ability of images.
In a stencil printing apparatus using a heatsensitive stencil sheet as the recording sheet, the heatsensitive stencil sheet is perforated by a perforating portion. The perforating portion comprises a thermal head and a platen roller. The front end of the perforated heatsensitive stencil sheet is attached to the peripheral surface of a cylindrical printing drum. By rotating the printing drum, the heatsensitive stencil sheet is wound around it, and being cut at a predetermined length.
Printing paper is pinched between the drum and a press roller and conveyed forward while the drum is rotating, so that printing is completed. A part of a peripheral wall of the printing drum is an ink-permeable printing area. In the printing drum, an ink-supplying device is disposed. In one operation, the heatsensitive stencil sheet should be cut in such a size as to cover the printing area in a circumferential direction of the drum after being wound around the drum. The length of the heatsensitive stencil sheet that is cut and wound around the drum for one operation is not measured actually, but is controlled according to the rotating number of a conveying means for the heatsensitive stencil sheet so that the printing area is covered.
Consequently, in such stencil printing apparatus, if the flange is improperly attached to the axial tube as stated above, the back tension acting on the heatsensitive stencil sheet increases more than necessary, thereby causing slipping in the perforating portion. Owing to this fact, the stencil sheet that is actually cut and wound around the drum for one operation may be shorter than the regular length. In such case, a part of the printing area of the drum, through which ink can be permeated, is exposed. Ink may leak from the portion, and soiling a printed matter or the press roller. Further, in the case where the slipping continues, an identical portion of the heatsensitive stencil sheet is repeatedly perforated. The portion deteriorates in printing-resistance strength. As a result, there occurs a drawback that the stencil sheet is tore in the portion when being printed. Further, when the slipping occurs, the distance between dots perforated by the thermal head decreases more than the regular distance. As a result, an image perforated on the heatsensitive stencil sheet shrinks. This brings about a drawback of deteriorating reproductive ability.
In view of the above-described various problems, it is an object of the present invention to provide a supporting device for supporting a rolled recording sheet, wherein a stable back tension can be attained during conveyance of the recording sheet even in the case where the flanges are attached imperfectly to the rolled recording sheet, so that the preferable reproductive results of images can be attained.